Preheat the oven to 250°F. Spray a 9x13-inch baking dish with non-stick cooking spray. Spread a layer of parchment paper onto the counter.
Into a large bowl, add corn cereal, rice cereal, pecans, and almonds. Stir to combine.
In a medium saucepan over medium heat, melt the butter.
Add the corn syrup, stirring to combine.
Add the brown sugar then continue cooking, stirring frequently until dissolved, about 5 minutes.
Remove from heat and carefully pour over the cereal mixture, stirring gently to fully coat.
Into the prepared baking dish, pour the coated mixture in an even layer.
Bake for 1 hour, stirring every 15 minutes.
Onto the parchment paper, spread the hot mixture in a single layer. Allow to cool completely then serve. Store any leftovers in an airtight container.